In general, an R--Fe--B type sintered magnet is composed of a main phase of Nd.sub.2 Fe.sub.14 B and an R-rich phase and a B-rich phase that serve as boundary phases. Various research has been conducted and proposals made in an effort to increase the Nd.sub.2 Fe.sub.14 B main phase, which affects the magnetic characteristics.
Meanwhile, R--Fe--B type sintered magnets may undergo dimensional changes, cracking, and distortion during sintering and magnet assembly, and the defect rate thereof can be as high as 10%, which is a major obstacle to reducing the cost of the magnets.
Methods known in the past for regenerating surplus and defective rare earth magnets include a wet metallurgical process in which the above-mentioned magnets are all chemically dissolved, and the rare earth components are extracted from the solution; a dry metallurgical process called smelting in which scraps of sintered magnets, defective magnets, or the like are melted and made into an R--Fe--B type alloy, and this alloy is reused as a starting material; and a method in which scraps or defective sintered magnets are reused as a mother alloy for melting.
A method for regenerating rare earth magnets has also been proposed in Japanese Laid-Open Patent Application Sho 58-049631, in which the impurity oxygen and carbon components in the defective magnets are mixed with calcium or Ca(OH).sub.2 and the mixture is subjected to calcium reduction decarburization to remove the oxygen and carbon, and in Japanese Laid-Open Patent Application Sho 61-153201, in which the carbon is first removed by heat treatment in a dehydrogenated atmosphere, after which direct reduction is performed with calcium to remove the oxygen.
The above-mentioned wet metallurgical process, which is a conventional regeneration method, is relatively advantageous in the case of an R--Co--based magnet which involves complicated steps and whose main structural component is a rare earth metal or a relatively expensive element such as cobalt, but in the case of an R--Fe--B type magnet, because the magnet contains about 65% iron, which is inexpensive, this process has little merit from a cost standpoint.
A dry metallurgical process, meanwhile, generates a large amount of slag during melting, and the rare earth elements are inevitably eluted into the slag, which requires that the rare earth metal components be recovered from the slag in a separate step.
Furthermore, the composition has to be readjusted in order to use re-melted magnets as a mother raw material for melting, and the components are difficult to control, among other disadvantages, and problems encountered with re-melting include the difficulty of removing oxygen down to the original level of the molten alloy.
The above melting methods essentially all involve regeneration into an alloying raw material, and do not allow for effective regeneration while leaving alone the texture of the sintered magnets, and particularly the main phase crystal grains, with which the magnetic characteristics will be enhanced.
Also, the above-mentioned methods for regenerating surplus or defective rare earth magnets by calcium reduction are aimed at the polishing dust, fragments, and chunks that are produced in the process of manufacturing rare earth magnets, and are substantially aimed at the polishing dust of SmCo-based magnets.